Method for increasing the burning rate of metal powders



Patented Sept. 7, 1954 METHOD FOR INCREASING THE BURNING RATE OF METAL POWDERS Mario D. Banus, Ipswich, and James J. McSharry,

Milton, Mass., assignors to Metal Hydrides Incorporated, Beverly, Mass., a corporation of Massachusetts No Drawing. Application July 9, 1952, Serial No. 297,994

1 Claim.

This invention relates to a method for increasing the burning rate of a pyrophoric metal powder with which hydrogen combines, such as powders of titanium, zirconium, etc. More particularly, the invention relates to the preparation of titanium metal powders which are suitable as a component of first fire mixtures.

First fire mixtures are used in incendiary bombs or shells. Such bombs usually are dropped from aircraft and comprise a metal casing containing a thermite mixture which it is desired to ignite. The first fire mixture is positioned adjacent the thermite mixture. A hammer is arranged within the bomb so as to be released to strike a percussion cap upon impact of the bomb with the ground or other target. When this happens the percussion cap ignites the first fire mixture which in turn ignites the thermite mixture.

Some titanium metal powders burn rapidly and are highly useful as a component of first fire mixtures while other titanium metal powders burn more slowly and are unsuitable as a component of first fire mixtures. The reason for this radical difference in the burning rate of different titanium metal powders has not been understood prior to the present invention. The problem has been particularly puzzling because it had been believed that powders having a low content of combined hydrogen should burn at a faster rate than those having a higher content of combined hydrogen. Nevertheless, some powders having a relatively high content of combined hydrogen have been found to burn more rapidly than others having a substantially lower content of combined hydrogen.

As a result of extensive research it has been discovered that there are several factors which affect the burning rate of pyrophoric metal powdersof titanium, zirconium and the like which combine with hydrogen, such as moisture content and hygroscopicity, metallic impurities, bulk density and hydrogen content. However, it has been discovered that when such powders contain substantial amounts of combined hydrogen their burning rates can be increased by heating them under a suificiently high vacuum at a temperature suificiently high to remove combined hydrogen without sintering the powders. The amount of combined hydrogen which can remain in a powder having a desired burning rate depends largely upon the bulk density of the original powder, the amount being greater for powders of low bulk density. Although the burning rate is affected to some extent by other factors, there is a predominating relationship between burning rate, bulk density and combined hydrogen content such that if two of these properties are known the other property can be predicted approximately. Thus, if the bulk density of a given powder is known, it can be predicted that the amount of combined hydrogen should not be greater than approximately a certain amount to provide a metal powder having a desired burning rate providing the heating under vacuum is conducted so that the powder is not sintered to increase its bulk density.

A fast burning titanium metal powder suitable as a component in first fire mixtures should have a bulk density not less than about 0.4 gram per cubic centimeter and not greater than about 1.7 grams per cubic centimeter. When the bulk density of the original powder is about 0.4 gram per cubic centimeter, the combined hydrogen content should not be greater than about 0.425 per cent by weight and should be about 0.025 per cent less for each 0.1 gram per cubic centimeter increase in bulk density of the original powder. Thus, for example, a titanium metal powder suitable for this purpose and having a bulk density of 0.7 gram per cubic centimeter should have a combined hydrogen content not greater than about 0.35 per cent while one having a bulk density of 1.2 grams per cubic centimeter should have a combined hydrogen content not greater than about 0.225 per cent.

When treating a titanium metal powder to increase its burning rate heating at a temperature of between about 500 C. to 600 C. has been found satisfactory under a vacuum of about 50 microns of mercury. Heating at such a temperature and under such a vacuum may be continued for the period of time necessary to reduce the amount of combined hydrogen to the desired amount without sintering the powder.

The relative burning rates 'of metal powders can be determined as follows. A cellulose acetate tube 10 inches long with an inside diameter of 0.250 inch and 0007:0001 inch wall thickness may be used. One end of the tube is sealed with 3 a small piece of Scotch tape. The tube is filled with the metal powder which has been dried within the hour for at least four hours at 120- 140 C. at a vacuum of 1-5 mm. of mercury, cooled under vacuum and stored in tightly closed containers until used. The tube is filled by means of a small funnel whose stem should fit inside the neck of the tube. The powder is poured into the funnel as needed by a small spatula or scoop with a minimum of agitation and vibration of funnel and, tube. When-the tube is-full, the. funnel is removed and the bottom of the tube tapped gently twice on the desk and the resulting cavity filled with additional powder. It is then laid on a surface of fire brick as gently as. possible and a small train of metal powder laid from the mouth out along the fire brick. This powder train can be lighted either with a. microburner or a small section of quick'match; laid.- across the powder train. The burning is timed with a stop watch which is started as the powder train burns to the mouth of the tube and is stopped: when.

the combustion front burns through the Scotch tape at the end of the tube. A least two.=determinations should be made which should check within 2--5 per cent. Instead of a cellulose acetate tube, a cardboard tube may be used. However, the burning rate in the acetate tube is less than that in the cardboard tube. For example, a. sample burning in. 135- seconds, in. the acetate tube will require about 180. seconds in the cardboard. tube. A titanium metal powder suitable for use as a component of. a first. firemixture should have a burning rate of not greater than about 135 seconds in the. acetate tube.

The. invention is illustrated further by the fol- 1owing specific example:

18.4. grams of; titanium metal powder was placed in a stainless steel boat and the boat placed in a quartz high capacity vacuum unit. A vacuum of less than one micron of mercury was obtained at room temperature. The temperature was then raised to 500 C. and maintained for 48. hours while maintaining a high vacuum. The unit was then cooled to room temperature under vacuum and the boat removed under nitrogen. The following table shows the burning time in a cardboard tube, the bulk density and the hydrogen content of the powder before and after degassing.

I'he method for preparing a fast burning titanium metal powder suitable as a component in first fire mixtures which comprises selecting a titanium metal powder having a bulk density betWeenIaboutD'A' and 1.7 grams per cubic centimeter and containing combined hydrogen in an amount greater than a predetermined amount,

said selected metal powder being capable of being heated; without sintering under a vacuum of at least. microns of mercury at a temperature between about 500 C. and 600 C. until the amount of combined hydrogen is not greater than a desired amount, and heating the selected metal powder at a temperature between about 500 C. and 600 6: while subjecting" it to a vacuum of about 50" microns of mercury until the amount ofcombined hydrogen isnot greater than said predetermined" amount, said predetermined amount being 0.425 percent, by weight when the bulk density of the original powderis 0.4 grams per cubic centimeter and being 0.025 percent less for each 011 grams per; cubic centimeter greater bulk density ofthe original powder.

References Cited in the file of this patent UNITED STATES" PATENTS Number. Name- Date 2,.10'7.27,9 Balkee rlr Feb. 8, 1938 2.205585% Knoll June 25. 194.0 2;.4=90,5.70 Anicetti Dec. 6.. 1949 2,537,068 Lillendahl Jan. 9,. 1951 2.640.770; Magram Jan. 2,1953 

